1. Field of the Invention
The invention pertains broadly to the field of laser gyroscopes. More particularly, the invention pertains to four-frequency laser gyroscope systems having effectively two laser gyroscopes operating simultaneously with a common propagation path.
2. Description of the Prior Art
The operation of a basic four-frequency laser gyroscope is described in U.S. Pat. No. 3,741,657 issued June 26, 1973 to K. Andringa and assigned to the present assignee. In such systems as described in the referenced patent, waves of four distinct frequencies propagate around a closed propagation path defined by three or more mirrors. Two of these beams circulate around the closed propagation path in the clockwise direction while the other two circulate in the counterclockwise direction. One of the clockwise beams and one of the counterclockwise beams are of a first polarization sense while the other one of the clockwise and the other one of the counterclockwise beams are of another polarization sense. For example, the first clockwise beam and first counterclockwise beam may be of right-hand circular polarization while the second clockwise and second counterclockwise beams may be of a left-hand circular polarization. The two right-hand circularly polarized beams may be, for example, of the highest two frequencies while the two left-hand circularly polarized beams may be of the lowest two frequencies.
Rotation of the laser gyroscope about its sensitive axis causes the two right-hand circularly polarized beams to become further apart in frequency than at the rest state while the two left-hand circularly polarized beams become closer together in frequency. The opposite frequency shifts occur for the opposite direction of rotation. As shown in the referenced path, the difference between the frequency shifts in the right-hand circularly polarized beams and the left-hand circularly polarized beams is directly proportional to the rate of rotation of the system. The time integral of this difference is directly proportional to the total amount of rotation about the sensitive axis.
Two separate means are provided within the propagation path for producing frequency splitting in order to maintain the beams of four separate frequencies. In the system described in the referenced patent, a crystal rotator provides a split between the average of the frequencies of the right and left-hand circularly polarized beams. This split is accomplished by the crystal providing a phase delay for circularly polarized waves that is different for one sense of circular polarization than for the opposite sense and is reciprocal. A Faraday rotator further provides the frequency split between the frequencies of the clockwise and counterclockwise beams of like polarization. The Faraday rotator is non-reciprocal providing different phase delays for waves of the same polarization states propagating in opposite directions.
Although the system of the referenced patent has been found to function quite well, it has been found desirable to provide still further improvements. For example, it is desirable to eliminate as much solid material from the propagation path as possible as presence of any solid material within the path provides scattering centers from which light may be undesirably coupled from one beam to another thereby inducing output frequency drift into the system. Furthermore, it is desirable to provide a laser gyroscope system in which the coupling between beams of the opposite sense of polarization at the output detector is substantially eliminated.